NBAA: Supersonic flight may be feasible – but can Earth stand it?

The dream of bringing back supersonic commercial air travel is closer to reality than it has been since the Aérospatiale/BAC Concorde was retired in 2003.

NASA, Lockheed Martin and business aviation start-ups Aerion, Boom Technology and Spike Aerospace are all pursuing faster-than-sound flight projects. Much effort is being expended to devise aircraft shapes capable of conquering that scourge of Mach-plus flight – the sonic boom – and lawmakers in Washington DC have even discussed changes to the law that bans supersonic flight over the continental USA.

Bans on overland supersonic flight played no small role in Concorde's ultimate failure as a commercial project. New York to London or Paris were that aircraft's essentially sole routes, leaving it as little more than a national vanity project – however much an icon of 20th Century engineering it was – for France and the UK as represented by Air France and British Airways.

So those who yearn for a return of the thrill and roar of Concorde can take heart in work by NASA and Lockheed Martin. The two expect to fly a demonstrator called X-59 QueSST from 2022, to measure communities' perception of the noise made by its boom-suppressing shape. That data is due to be delivered to the US Federal Aviation Administration and ICAO in 2025, so they can in turn devise new rules, based on perceived sound levels, that may well enable supersonic flight over land.

Meanwhile, Reno, Nevada-based Aerion is thought to be readying a launch of its planned AS2 supersonic business jet, possibly by mid-2020. A feasibility study of the project being carried out by Lockheed Martin – which is contracted to help design and build the AS2 airframe – is expected to complete around year-end. Much hinges on GE Aviation, which is studying engine possibilities, based in part on technology that features in the subsonic CFM56, built by the GE-Safran partnership CFM International and which powers many Airbus A320s and all Boeing 737s.

Aerion is understandably bullish; US fractional operator Flexjet has signed a letter of intent for 20 of the three-engined, $120 million, Mach 1.4 aircraft. Scheduled for first flight in 2023, AS2 will, chief commercial officer Ernest Edwards told FlightGlobal earlier this year, be able to fly at M1.2 without making a sonic boom.

Perhaps even more ambitious is Colorado-headquartered Boom. In a Farnborough air show presentation this July, the company said it expected by end-2019 to fly a supersonic demonstrator for a future M2.2 airliner whose shape resembles Concorde's.

If realised – and despite delays to the demonstrator programme, Boom was, at Farnborough, still talking entry into service in 2025 – its 55-seater would outrun Concorde. Carriers including Virgin Atlantic and Japan Airlines are sufficiently impressed to have made commitments for up to 76 aircraft in an all-business-class configuration.

Spike's business jet plan calls for an 18-passenger jet capable of M1.6. A year ago, the company made flight tests of a scaled proof-of-concept demonstrator, to validate low-speed aerodynamics. Chief executive Vik Kachoria believes the market for supersonic travel will be 13 million passengers per year – many of them middle managers who fly a lot and value their time, rather than just billionaires – once flights start in 2025.

In May this year, Kachoria told Bloomberg Television that his company's $125 million aircraft would be flying by the end of 2023 – marking, perhaps coincidentally, the 20th anniversary of Concorde’s retirement.

None of these projects is sticking to the schedules envisioned earlier this decade, not unusual for aerospace and certainly not for concepts as technically and financially challenging as supersonic flight.

But while an optimistic reading of these efforts suggests a return to supersonic flight is inevitable, noise and economics are not the only obstacles presented by the sound barrier. Another factor raises a spectre more ominous than noise on the ground or high ticket prices – concerns about emissions and climate raise the question: should any of these aircraft fly at all?

HIGH COST OF SPEED

A July 2018 report by the International Council on Clean Transportation concludes that new supersonic aircraft could burn five to seven times as much fuel as standard aircraft – and exceed CO2 emission limits by 70%. The ICCT, an independent non-profit organisation that provides "unbiased research and technical and scientific analysis to environmental regulators", based its work on publicly available information from Boom. Business jets, it says, "are a small contributor to overall emissions, [so] we decided to focus on commercial supersonics in this analysis".

The ICCT found that supersonic flight would consume some 600kg (1,320lb) per passenger on a New York JFK to London Heathrow flight – more than twice as much fuel as burned to transport subsonic business-class passengers on an Airbus A321LR, and some six times as much as for economy class.

Going supersonic from Los Angeles to Sydney approaches 1,500kg per passenger by the ICCT's reckoning – around three times as much as a subsonic business-class passenger accounts for on a Boeing 787-9.

And, the ICCT notes, assumptions about real-world supersonic performance are likely to be overestimates.

The ICCT concludes that short-term commercial supersonic aircraft are "unlikely to comply with existing [emissions] standards for commercial aircraft". It recommends two ways forward. One is for manufacturers to refocus their efforts on advanced, clean-sheet engines more likely to meet existing standards. Or, policymakers could set new standards for supersonics to allow increased pollution compared with new subsonic aircraft.

That latter course is problematic, though. Says the ICCT: "The share of CO2 emissions attributable to international aviation is expected to increase from 1.4% today to between 7% and 14% of a Paris-compatible global carbon budget by 2075. The introduction of new supersonic aircraft opens up the potential for even larger increases.”

Perhaps foretelling political battles over the reintroduction of supersonic flight, the ICCT adds: "The social acceptability of this increase, and therefore the public's support for supersonic travel, remains to be seen."

Brussels-based research and policy group Transport & Environment has raised similar concerns. Writing to European government ministers for climate and transport, T&E says: "Modest gains in flying time to be enjoyed by a very privileged few will be far outweighed by the increased environmental impact of supersonic flight to the extent that they could endanger meeting the [1.5˚C global warming] goals of the Paris Agreement."

T&E’s own report concludes: "Given the expected impact of supersonic, it's difficult to see how its deployment can be reconciled with this [Paris] target."

T&E aviation manager Andrew Murphy tells FlightGlobal that if supersonic aircraft programmes are to be financially viable, large numbers of aircraft are on the cards – some 500 business jets, in his estimation. If that represents new demand, he says, then supersonics mean new pollution.

In any case, he sees no reason to assume that fuel burn will be better than Concorde's. And, because they fly higher than ordinary airliners, supersonics would have significant non-CO2 climate change effects. In short, says Murphy, a small number of aircraft can have a big environmental impact.

UNKNOWN QUANTITY

The actual emissions performance of any of the proposed supersonic aircraft remains an unknown. Boom global policy director Eli Dourado disagrees with the ICCT analysis, telling FlightGlobal: "Our airliner's fuel burn per seat mile will be similar to today's subsonic business class. The analysis also neglects to mention that starting in 2020, all carbon emissions growth in international aviation will be fully offset under ICAO's CORSIA [Carbon Offsetting and Reduction Scheme for International Aviation] framework.

"Combined with the growth of alternative fuels, we believe emission reductions and faster flight are not only both realistic but very achievable."

T&E is adamant that that there should be no "boutique" standards for supersonics. In any case, Murphy stresses, the Paris climate accord demands every sector cut emissions – a principle that undermines even the theoretical benefits of offsetting.

Offsetting, or paying other sectors to cut emissions when your sector cannot or will not, has always been problematic, he says; monitoring is dubious, and in any case it is rarely clear that an offsetting scheme is not paying to encourage reductions that would have happened anyway. And, he adds, aviation emissions in Europe have been rising by 6-8% per year for the past few years.

Kachoria says: "It is completely unacceptable to advance technology or transportation at the detriment or the environment or the community." And, he adds: "We welcome discussions with environmental groups to understand concerns and considerations important to them."

As for noise, he says Spike's S-512 supersonic jet will feature "medium bypass-ratio engines with lateral shielding and computer-optimised accelerate/climb rates to mitigate airport community noise", while "mitigating the sonic boom by aerodynamic design of the nose cone, wing, engines and empennage". On emissions, he is perhaps less reassuring: "Engines are 80% more efficient than they were in the 1960s and 75% quieter. Aviation accounts for 2% of global emissions."

Cost, noise and technology are still unknowns, so for the moment, supersonic air travel remains a prospect rather than a certainty. But clearly, we can expect political battles over its future – if it has one.